This invention relates to an apparatus for hydrostatically extruding thermoplastic polymers in a solid state and in particular to the hydrostatic extrusion of orientable thermoplastic polymers.
The thermoplastic polymers are extruded in a solid state in an apparatus with an extrusion zone having converging walls, a converging cross-section and a diverging geometry whereby the polymer is substantially simultaneously elongated both circumferentially and axially.
It is well known that the physical and mechanical properties of semi-crystalline thermoplastic polymers can be improved by orienting their structures. Polymer processing methods, such as drawing, blow molding, injection molding and the like have all been used to fabricate articles of thermoplastic polymers having oriented structures.
In recent years, extensive study has been directed to methods of deforming the thermoplastic polymers in a solid state. In these methods, the polymer is mechanically deformed to obtain a desired uniaxial or biaxial molecular orientation. The polymer may be drawn, extruded or processed at temperatures within the range from the glass transition temperature to temperatures just below the crystalline melt temperatures of the polymers. In the case of stereoregular polypropylene, the polymer must be processed at temperatures as low as 0.degree. C. (32.degree. F.). Products such as strip, tubes, rods and shapes, usually having predominantly unidirectional orientation, have been fabricated by such processing methods. The extrusion methods and apparatus used for processing the polymers are similar to those used in the metal industry. Short tubular articles with high axial elongation and low circumferential elongation, for example shotgun shells, have been produced by solid state extrusion.
One method for processing a polymer is described by Robert A. Covington, Jr. et al in U.S. Pat. No. 3,205,290 entitled "Method of Making Tubing for Cartridge Casings and the Like." In the method, a molten polymer, for example polyethylene or polypropylene, is formed into a thick-walled tubular preform or billet. The billet is processed in a two-step process into a short, thick-walled tubular article having one closed end. Initially, the billet is expanded circumferentially by an average of about 40 to 50 percent by forcing it onto a solid mandrel. Circumferential elongation refers to the expansion of the median circumference of the billet. The expanded billet on the solid mandrel is then forced through a drawing die to elongate the expanded billet in an axial direction while the circumferential elongation remains constant. The axial elongation can be as much as 350 percent, resulting in a predominantly axial orientation.
The Covington et al method does not allow a circumferential expansion of at least 100 percent. If such large circumferential expansions were attempted, the billet would buckle or collapse in an effort to push it over the mandrel. If large circumferential deformations of 100 percent or more could be made by Covington et al, the deformations would be tensile in nature because the billet would be drawn over the mandrel. Drawing the billet over the mandrel would result in non-homogeneous deformation of the polymer structure.
U.S. Pat. No. 3,198,866 to R. A. Covington et al entitled "Method and Apparatus for Producing Plastic Tubular Members" is directed to a continuous method for producing tubular members. In the method, thick-walled, bored slugs of a thermoplastic polymer, polyethylene having a crystallinity of 60 to 85 percent, are forced over a mandrel by ram pressure.
The patent contends that the molecular structure of the polymer is oriented both longitudinally and transversely. However, the apparatus of Covington et al is designed to prevent any significant increase in the outside diameter of the slug, i.e. the polymer is not expanded circumferentially into a conduit having a larger outside diameter than the outside diameter of the slug. Since the slug is increased in length and the wall thickness is decreased but the outside diameter is not increased, the polymer is highly oriented in the longitudinal direction but is not highly oriented in the circumferential direction. There is little median circumferential elongation therefore there is little, if any, improvement of the average properties in the circumferential direction.
Another process used to produce oriented shotshells is described by Donald Urquhart Findlay et al in U.S. Pat. No. 3,929,960 entitled "Method for Producing Oriented Plastic Shotshells." The method is directed to making an oriented polyolefinic shotshell with an axial tensile strength between about 1400 and 2100 kilograms of force per square centimeter (20,000 and 30,000 pounds per square inch) and a circumferential tensile strength between about 387 and 600 kilograms of force per square centimeter (5,500 and 8,500 pounds per square inch). A polyolefinic blank which is 2.54 centimeters (1 inch) in length and having a wall thickness of 1.06 centimeters (0.42 inch) is heated to a temperature between 27.degree. C. and 115.degree. C. (80.degree. F. and 240.degree. F.) and is placed on a solid movable mandrel. The blank is moved into a die cavity. A ram forces the blank over the mandrel in a back extrusion to reduce the blank wall with very little, if any, expansion of the outside diameter of the blank.
The method of Findlay et al limits the circumferential expansion of the polymer, hence limits the circumferential deformation of the polymer structure. Since the axial elongation is high, the molecular structure is highly oriented in the axial direction. The structure, comprised of spherulitic crystalline aggregates, is highly elongated axially but with very little elongation circumferentially.
The indirect extrusion method of Findlay et al limits the expansion of the outside diameter of the blank to below 25 percent which is well below the minimum circumferential expansion achieved in the method of the invention hereinafter described.
As noted by Robert Shaw in U.S. Pat. No. 3,714,320 entitled "Cold Extrusion Process", polymers, particularly stereoregular polypropylene, can be fabricated by various methods such as rolling, forging, swaging and peening at temperatures below the crystalline melt temperature. Shaw teaches that cold extrusion of polymers has limited application because excessive heat is generated during large deformations thereby increasing the temperature of the polymer to its melting temperature. Shaw attempts to overcome the problem of extruding polymers by cooling them to temperatures as low as 0.degree. C. (32.degree. F.). If necessary, the extrusion apparatus can also be cooled to low temperatures. Forward extrusion results in the conversion of rod-like shapes into rod-like extrudates of various cross-sectional shapes having a generally reduced cross-sectional area. It is apparent that Shaw does not envision making circumferentially elongated pipes and conduit by extrusion since he teaches that tubes or pipes may be formed by a known manner similar to the so-called Mannesmann method in which a mandrel is placed inside a tube and a rolling or hammering force is applied to the outside surface. Back extrusion can be used to produce cup-like shapes.
Shaw's teaching is diametrically opposed to an extrusion process in which a thermoplastic polymer is heated to a temperature which is between its 4.64 kilograms force per square centimeter (66 pounds per square inch) heat deflection temperature and its crystalline melt temperature for extrusion through a die configuration which will substantially simultaneously elongate the polymer circumferentially and axially.
In the limited application of Shaw's process to extrusion in which he teaches that the polymer must be cooled to low temperatures, it would require excessively high pressures, on the order of 10 times as great as those required to warm extrude the polymer, in order to extrude the cooled polymer into a tube comprised of highly oriented polymer. The use of excessively high pressures applied to a relatively strong material would result in stick-slip, high strain rate, high energy extrusion and periodic generation of high temperatures at which the polymer would melt. When a polymer melts, the crystallinity and orientation in the polymer are adversely affected and the product is damaged beyond use. Therefore, a polymer processed according to Shaw could not possibly have a structure comprised of platelet or wafer-like, radially compressed spherulitic crystalline aggregates oriented both circumferentially and axially and having improved circumferential properties.
Long, thick-walled high strength tubular polymer products, such as high pressure hoses, tubes and pipes have been produced by plasticating extrusion of fiber reinforced plastics and medium pressure tubes by plasticating extrusion methods.
One such method for producing medium pressure thermoplastic pipe having a diameter as large as 152.4 centimeters (60 inches) and a wall thickness of over 5.08 centimeters (2 inches) is described in U.S. Pat. No. 3,907,961 to Guy E. Carro entitled "Flexible Cylinder for Cooling and Extruded Pipe." The pipe can be made by either screw extrusion or impact extrusion. In either case, the thermoplastic polymer is heated to a molten state and is extruded through a conical shape passage onto a flexible mandrel. A cooling medium is provided to cool the surface of the pipe to a solidified state. The polymer is extruded in the molten state and the resultant pipe has an unoriented structure.
A method for producing high pressure pipe is described in U.S. Pat. No. 4,056,591 to Lloyd A. Goettler et al, which is directed to a process for controlling the orientation of discontinuous fiber in a fiber reinforced product produced by melt or plasticating extrusion. The fiber-filled matrix is extruded through a diverging die having a generally constant channel. The walls of the die may taper slightly so that the area of the outlet of the die is larger than the area of the inlet of the die. The amount of orientation of the fibers in the hoop direction is directly related to the area expansion of the channel from the inlet to the outlet of the channel. The product produced is a reinforced hose containing fibers which are oriented in the circumferential direction to improve the circumferential properties. While the fibers may be oriented, the polymer is unoriented since it is processed in a molten state.
Biaxially oriented containers, such as bottles used in the soft drink industry are made by a melt extrusion-stretching or injection molding-blowing expanding process.
One such process in which a biaxially oriented hollow article having good transparency and strength and made from polypropylene is processed by the method described in U.S. Pat. No. 3,923,943 to Fumio Iriko et al entitled "Method for Molding Synthetic Resin Hollow Articles." In the method, the initial step is the production of a parison by injection molding. The parison is expanded by stretching in contradistinction to being expanded by compressive forces therefore the structure is non-homogeneously deformed and is susceptible to the formation of microvoids thereby decreasing the density of the polymer typically about 0.5 percent.
A second method employed to produce a biaxially oriented container is described by Fred E. Wiley et al in U.S. Pat. No. 3,896,200 entitled "Method of Molding Biaxially Oriented Hollow Articles." A parison is held in constant tension and is stretched in the axial direction before or as it is expanded radially into a cavity.
Still another method for producing containers which have clarity is described in U.S. Pat. No. 4,002,709 to Larry P. Mozer entitled "Controlled Air in Polyester Tube Extrusion for Clear Sealable Parison." In the process a polyester, for example polyethylene terephthalate, is melt extruded into a clear thick-walled tubing which is then heated and blown into a container. The polyester is in an amorphous state as evidenced by the clarity of the tubing.
The containers in the above processes are produced by stretching the polymer, typically over 250 percent. Such large stretching deformations result in non-homogeneous deformation of the structure thereby damaging the spherulitic crystalline aggregates, causing the formation of microvoids and the enlargement of microvoids already present in the polymer. The density of the polymer is decreased and the microstructural sensitive properties, such as stress whitening and low temperature brittleness are not eliminated.
It is desired to provide an apparatus whereby a thermoplastic polymer is compressed as it is processed whereby the problems of non-homogeneous deformation and the associated defects are suppressed and a biaxially oriented spherulitic crystalline aggregate structure subtantially free from such defects is produced.
The prior art processes described above, by which tubular products consisting essentially of thermoplastic polymers are produced are incapable of and cannot be adapted to expand a polymer by at least 100 percent in the circumferential direction in a compression-type deformation. Prior art processes for producing hoses or elongated tubular products are directed to melt or plasticating extrusion processes which result in the production of non-oriented products. Prior art processes for producing large diameter containers are directed to stretching or tensioning processes in which a polymer is expanded at least 100 percent in the circumferential direction. None of the prior art processes described above produces a conduit consisting essentially of a crystalline thermoplastic polymer which is expanded at least 100 percent in the circumferential direction and is expanded at least 50 percent in the axial direction.
It is the object of this invention to provide an apparatus for hydrostatically extruding thermoplastic polymers in a solid state wherein the polymers are compressed when extruded through an extrusion zone which may be bell-shaped and which has converging walls, a converging cross-section and a diverging geometry whereby the problems of non-homogeneous deformation and associated defects are suppressed and a biaxially oriented spherulitic crystalline aggregate structure substantially free from such defects is produced.
Other objects of this invention will appear more clearly from the following detailed description and drawings.